Because of high ion density of produced plasmas the arc discharges represent very powerful tools in the plasma processing technology. Arc sources are used for generation of reactive plasmas in a working gas, they can produce plasma which contains particles of sputtered and/or evaporated electrodes, or chemical compounds of these particles with the working gas. Properties of arc discharges depend on energies and densities of the generated ions and electrons, which are affected by the pressure of the working gas. A wide variety of gas plasmatrons use arc discharges at atmospheric pressures, because of simple arrangement without vacuum pumps. However, generation of an arc based on a non-isothermal plasma with controllable ion energies requires low gas pressures. Different types of arc discharges are used for this purpose. Most of them are generated by direct current (DC), high power generators. A typical low pressure source for plasma processing is a cascaded arc (European patent 0297637) which produces a flow of an active plasma in a working gas. Metal ions in plasmas are generated usually from an electrode which is sputtered and/or evaporated by the arc discharge, see D. M. Sanders et al., IEEE Trans. Plasma Sci. 18, 883-894 (1990). The eroded electrode is usually a planar cathode in the DC arc circuit with an electrically grounded anode. The cathode is water cooled and the content of undesirable micro droplets--"macroparticles" present in the evaporated cathode material is reduced by steering the cathode spot motion on the cathode surface. In recent cathodic arc sources the macroparticles are filtered by an auxiliary magnetic field filter (U.S. Pat. No. 5,279,723). The content of macroparticles is usually lower when the consumable electrode is heated up to melting temperature. It is also possible to utilize arc arrangements with a consumable anode instead of the cathode, see e.g. M. Mausbach et al., Vacuum 41, 1393-1395 (1990). Cold consumable cathodes are of limited size and require the magnet filtering, which limits possibility of scaling these sources up. Consumable electrodes containing liquid metal crucibles can be installed only in restricted positions. Most arc sources require an additional switch to start the discharge.
A generation of an arc discharge is very efficient in hollow cathodes. The principle of the hollow cathode gas discharge generated by a direct current has been reported first by F. Paschen, Ann. Physik 50, 901-940 (1916). From that time a great number of investigations about this discharge have been reported, see reviews e.g. J-L. Delcroix and A. R. Trindade, Advances in Electronics and Electron Phys. 35, 87-190 (1974), M. E. Pillow, Spectrochimica Acta 36B, no.8, 821-843 (1981), and R. Mavrodineanu, J. Res. Nat. Bureau of Standards 89, no. 2, 143-185 (1984). In DC hollow cathodes an arc discharge can be generated at high DC power. The wall of the hollow cathode must be heated up to high temperatures strongly enhancing thermionic emission of electrons. Moreover, a substantial portion of ions is produced by an erosion of the hollow cathode surface. At these conditions the DC current in the arc circuit grows up rapidly, while the voltage at the cathode falls down to values of the order of the minimum ionizing or minimum exciting potential of the gas or metal vapor. The arc is a self-sustained discharged capable of supporting large currents by providing its own mechanism of electron emission from the negative electrode (see "Handbook of Plasma Processing Technology" ed. by S. Rossnagel et al., Noyes Publ. 1990, Chapter 18 by D. Sanders). Until this condition is not reached the discharge in the hollow cathode cannot be assumed as an arc. It is rather a normal or anomalous glow, even if some parts of the cathode walls are hot, particularly in cathodes fabricated from a thin metal sheet.
Because of high production of electrons even in glow regimes the hollow cathodes have been used since 1971 as both an electron source and the working gas ionization source in plasma processing devices for plasma assisted anode evaporation (se e.g. U.S. Pat. No. 3,562,141). Since 1983 hollow cathode glow discharges generated by alternating current (AC) generators have been developed. Typical frequency of AC generators for this purpose is between 100 kHz and 100 MHz. In particular the radio frequency generators (RF-13.56 MHz and its harmonics) are often used in plasma processing (see e.g. C. M. Horwitz, Appl. Phys. Lett., vol. 43, 1983, p.977, and U.S. Pat. No. 4,521,286). A variety of hollow cathode arrangements were developed using this principle. Hollow cathode systems differ mainly in arrangements of electrodes, the inflow of working gases, etc. A hollow cathode principle can be used for an enhancement of plasma chemical vapor processing in a plane parallel arrangement of processed planar substrates which are at the cathode potential (European patent 0 478 984 A1). A cylindrical RF hollow cathode was used for plasma chemical vapor processing (Czech Patent 246,982/PV 4407-85) and for sputtering of the cathode and deposition of films inside hollow substrates and tubes (Czech Pat. Appl. PV3925-90). In RF generated hollow cathodes an anode is the RF plasma itself (a virtual anode) which is in contact with a real counter electrode connected to the RF generator (Bardo et al., J. Non Cryst. Solids 97/98, 281 (1987). The multiple cylindrical RF hollow cathode (19 tubes together) with closed bottom parts in a multi-cusp magnetic field has been used as an effective 175 mm diameter ion beam source, see C. Lejeune et al., Vacuum 36, 837 (1985) and in French Patent Application No. 85 06 492 (1985). The multiple RF hollow cathode (5 holes) in a linear distribution has been reported by A. M. Barklund et al., J. Vac. Sci. Technol. A9, 1055 (1991), (see also Czech Patent 246,982). The linear array of 64 DC or AC powered cylindrical hollow cathodes in an axial magnetic field providing about 40 cm long distributed discharge has been reported recently by A. Belkind et al., ICMCTF'94, poster session, San Diego 1994 (will appear in Proceedings in Surface Coat. Technol. (1994)). In this work an axial magnetic field of 0.025 Tesla was used to extract the plasma beyond the confinements of hollow cathodes. Effects of magnetic fields of different inductions have been often reported in hollow cathodes, see e.g. review by K. H. Schoenbach, invited paper at ICPIG 21, Bochum 1993, Proc. III, pp. 287-296. Because of a magnetic confinement effect the magnetic fields are often used for low pressure discharge regimes in hollow cathodes. Most of the systems mentioned above use glow discharge in a cylindrical hollow cathode. At sufficiently high power in an arc regime the production of metal ions from the hollow cathode wall is enough for a self sustained discharge without any other working gas (L. Bardo et al., Swedish Patent Application No. 9303426-2, International Patent Application PCT/SE9/00959). An arc hollow cathode regime in gas which contains a reactive component can be used for very high rate reactive deposition of films based on an enhanced production of the cathode metal particles. This deposition can be even faster than corresponding non-reactive deposition of pure metal film (see H. Barankova et al., Proc. 10th Symp. on Plasma Processing, Electrochem. Soc. Spring Meet., San Francisco 1994, Proc. Vol. 94-20, G. S. Mathad and D. W. Hess, eds., pp. 580-591). Most of the hollow cathode arrangements utilize cylindrical cathodes or small plane parallel cathode disks with a cylindrical anode around them. In general, the hollow cathode systems are of very limited dimensions. This can be an advantage for inside tube plasma processing (H. Kawasaki et al., Mat. Sci. Engineer. A140, 682 (1991)). Similarly to filtered arc discharges the discrete arcs produced by small size hollow cathodes are of limited possibility of scaling up. Linear arrays of multiple cathodes cannot produce uniform linear discharge and they depend on function of each particular discharge.